1. Field of the Invention
The present invention relates to an imaging apparatus and an ophthalmic apparatus, and more particularly relates to an imaging apparatus and an ophthalmic apparatus for tomographic imaging of a fundus or skin or the like using optical coherence tomography.
2. Description of the Related Art
In recent years, apparatuses for optical coherence tomography (referred to below as “OCT” below) using optical interference technology with low coherence light have been put into practice. OCT apparatuses are useful in the medical field, in particular in ophthalmology. They are apparatuses with which it is possible to obtain a tomographic image of the fundus/retina, and are becoming indispensable for the diagnosis of diseases of the fundus.
The following is a simple explanation of the principle of OCT. First, low coherence light is divided into reference light and measurement light. The measurement light is irradiated onto the measurement object, and by causing interference between the reference light and the returning light reflected from the tomographic imaging object region, it is possible to obtain a tomographic image of the measurement object. There are two OCT principles: TD (time domain) OCT and FD (fourier domain) OCT. In the FD-OCT method, the tomographic image is obtained by a Fourier transformation of the obtained interference signal. With this method, the tomographic image can be obtained faster than with the TD method, so that this method is presently the mainstream.
In recent years, there have been efforts to increase the resolution when obtaining the tomographic images in order to improve the image quality of the tomographic images. The resolution in OCT is classified into longitudinal resolution, which is the resolution in the optical axis direction of the measurement light, and lateral resolution, which is the resolution in a direction perpendicular to the optical axis direction. The resolution in longitudinal direction is important in order to identify the layer structure in fundus tomographic measurements using OCT, and the thickness of the layers is very important in judging eye diseases.
In OCT, the longitudinal resolution is determined mainly by the characteristics of the light that is used for the measurement. When the wavelength spectrum of the light has a Gaussian distribution, the longitudinal resolution can be expressed by the following equation:
                              [                      Eq            .                                                  ⁢            1                    ]                ⁢                                                                                                l          c                =                                            2              ⁢                              ln                ⁡                                  (                  2                  )                                                      π                    ⁢                                    λ              0              2                                      Δ              ⁢                                                          ⁢              λ                                                          (        1        )            
Here, lc is the longitudinal resolution expressed as the full width at half-maximum of the coherence function. λ0 represents the center wavelength of the light, and Δλ represents the wavelength width of the light. In Equation (1), it is assumed that the wavelength spectrum has a Gaussian distribution. When light is used whose spectrum does not have a Gaussian distribution, then the longitudinal resolution will be poorer than expressed by (1), but since the center wavelength λ0 and the wavelength width Δλ undergo the same change, it does not lose its generality. With Equation (1), it can be seen that the longitudinal resolution can be improved by the two measures of:
(a) making the center wavelength λ0 shorter, and
(b) widening the wavelength width Δλ of the light. In OCT for ophthalmology, the infrared wavelength region (near 850 nm) is used. Due to optical absorption of visible light with the retina and absorption by the water in the vitreous body, there is a limit on the short-wavelength side. Consequently, since the wavelength range that can be used in ophthalmic OCT is limited on the short-wavelength side, it is difficult to increase the longitudinal resolution by making the center wavelength λ shorter. Also, since it is necessary to avoid absorption losses by the vitrous body located in front of the fundus, in order to let the measurement light reach the fundus, there is also a limit on the long-wavelength side. Consequently, an increase of the longitudinal resolution is realized by (b) widening the wavelength width Δλ of the light while giving consideration to these limits. Actually, since large-bandwidth low-coherence light is being put into practice in recent years, there is a discussion on improving the longitudinal resolution through (b) and its clinical merits (see for example “Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation”, OPTICS EXPRESS Vol. 12, No. 11, 31 May 2004, pp. 2404-2422). The following is a discussion on dispersion compensation. In OCT, it is necessary to match the dispersion characteristics between the reference light path and the measurement light path. This matching of the dispersion characteristics is called “dispersion compensation.” FIG. 4 is a diagrammatic view showing an intensity profile in the depth direction by OCT for the case that dispersion compensation is performed and the case that dispersion compensation is not performed. The dotted line diagrammatically shows the profile that is obtained when no dispersion compensation is performed, whereas the solid line diagrammatically shows the profile when dispersion compensation is performed. FIG. 4 shows that when the dispersion compensation is insufficient, the intensity of the coherence function, which represents the resolution in the depth direction, drops, and the full width at half-maximum widens, so that the longitudinal resolution deteriorates.
Japanese Patent Laid-Open No. 2007-267927 discloses the use of water for dispersion compensation in OCT. A container that is filled with a medium of at least 70% water content is placed on the reference light path side. Through this medium, the characteristic feature is attained that the influence of the dispersion due to the measurement object can be suppressed.
Moreover, Japanese Patent Laid-Open No. 2006-162366 discloses an OCT apparatus having a plurality of light sources of different wavelength ranges, and splitting the reference light path in correspondence with the measurement light. By using a plurality of wavelength ranges, it is possible to obtain, at the same time, tomographic images of different depth regions.
In order to increase the longitudinal resolution using light of a large bandwidth in OCT, it is important to perform dispersion compensation across the entire used wavelength range. Since the dispersion characteristics of the measured object differ for each wavelength, there is the problem that the dispersion compensation becomes more difficult as the wavelength range becomes wider, and there is the risk that this impedes the improvement of the longitudinal resolution.
In “Ultrahigh-resolution, high-speed, Fourier domain optical coherence tomography and methods for dispersion compensation”, OPTICS EXPRESS Vol. 12, No. 11, 31 May 2004, pp. 2404-2422, the dispersion compensation is performed with a plurality of glass materials for OCT using light of a large bandwidth. The dispersion characteristics of the materials water and glass differ greatly in the long wavelength region (the wavelength range of about 900 nm to 950 nm), so that there is the problem that it is difficult to perform the dispersion compensation with water over a large bandwidth.
In Japanese Patent Laid-Open No. 2007-267927, a dispersion compensation adapted to the OCT measurement object is carried out with water. However, with a dispersion compensation with water, there is the possibility that problems may occur in steady use with regard to temperature dependency and product quality.
Moreover, in Japanese Patent Laid-Open No. 2006-162366, it is possible to obtain, at the same time, tomographic images of different depth regions by using a plurality of wavelength ranges, but there is no dispersive material up to the region of interest of the tomographic images of the measured object, so that there is the problem that no consideration at all is given to dispersion compensation.
In view of the above problems, the present invention provides a technology for realizing dispersion compensation that is adapted to the dispersion characteristics of light of a large bandwidth.